1. Field of the Invention
The present invention relates to a method for manufacturing a zeolite membrane for separating a mixed fluid and to a zeolite membrane.
2. Description of Related Art
Zeolite is used as a catalyst, a catalyst carrier, an adsorbent, and the like. In addition, a zeolite membrane formed on a surface of a porous substrate made of metal or ceramic is used as a gas separation membrane or a pervaporation membrane by using a molecular sieve function of zeolite. As to membrane separation with a zeolite membrane, a membrane used for dehydration of an organic solvent or biomass ethanol has been put to practical use in recent years.
In order to enhance the performance for using a zeolite membrane as a separation membrane, the improvement in separation factor and permeance is desired. That is, important issues for practical use of the zeolite membrane are compatibility of separation performance and permeation property. However, these are incompatible, and, for example, it is not easy to enhance the permeance without lowering the separation factor.
There is disclosed a method for improving permeation property by manufacturing a thin zeolite membrane uniformly. For example, there is disclosed a method for forming a thin and uniform zeolite membrane by, after impregnating a porous substrate with a certain substance, attaching a seed crystal on the surface (see JP-A-2008-18387 and JP-A-2004-344755). In addition, there is disclosed a method using two kinds of impregnation materials (impregnation material, preliminary impregnation masking material) (see EP 1163046 A1).
However, in the methods of JP-A-2008-18387 and JP-A-2004-344755, the depth control of a substance impregnating to the porous substrate is difficult, and the uniformity of the zeolite membrane formed on a surface of the porous substrate is not sufficient. In the method of EP 1163046 A1, in the case of impregnating a support with an impregnation material with forming a preliminary impregnation masking layer, the subsequent complete removal of the preliminary impregnation masking layer needs much time. For example, in the case of polymethylmethacrylate resin (polymethylmethacrylate, PMMA) cited as a preferable material for the preliminary impregnation masking layer, one week is required by acetone according to J. Hedlund et al., Journal of Membrane Science 222 (2003), 163-179. In the case of removing the preliminary impregnation masking layer, the support has to be impregnated with a substance insoluble in a solvent (e.g., acetone) used for removing the preliminary impregnation masking layer as the impregnation material. In addition, the impregnation material with which the support is impregnated should have a melting point not lower than zeolite synthesis temperature. Since many kinds of low polar polymers are hydrophobic, when it is used as the masking polymer (impregnation material) as in the EP 1163046 A1, the surface has a negative electric charge to repel a solvent such as water upon applying a seed crystal, and sufficient application on the surface of the porous substrate becomes difficult. Therefore, the “Journal of Membrane Science” 222 (2003) discloses a method where the electric charge is removed by applying a cation polymer on the surface of the porous substrate before the application of a seed crystal to subsequently apply a seed crystal. However, such a method leads to increase in production cost because the operation becomes complex to a large extent upon manufacturing a membrane.